Typically in coherent-demodulation Systems, e.g. with quadrature amplitude modulation (QAM), local voltage controlled oscillators (VCO's) are used and driven in such a way as to generate an oscillation in phase and frequency coherence with the carrier of the received signal. The possibility of using free local oscillators having a frequency close to the frequency of the carrier would mean a remarkable advantage in terms of cost. However, such oscillators have tolerances different from zero on the nominal frequency and drift both with heat and aging. This requires a device which recognizes the frequency and phase difference between the oscillation generated by the local oscillator and the carrier of the signal and is capable of correcting the effects on the signal to be processed. The devices of this kind (phase and frequency locked loops) are characterized, inter alia, by the lock-in band, i.e. by the amount of frequency errors which they are able to recognize and correct. They belong to two main categories:
devices that make use of estimates of the received data (having a lock-in band smaller than one-eighth of the symbol frequency); PA1 devices that do not make use of estimates of the received data (having lock-in bands even greater than the symbol frequency but are sensitive to selective fading). PA1 A) a subsystem for recovering phase and frequency of the carrier, PA1 B) a subsystem for reconstructing the synchronism frequency capable of driving the signal sampling circuit, and PA1 C) a subsystem composed of filters. PA1 a) a system for recovering the carrier in turn comprises two subsystems: a quadricorrelator (subsystem A1 in FIG. 1) for recovering the carrier frequency and a loop (subsystem A2 in FIG. 1) for jointly recovering phase and frequency of the carrier that uses the estimated data. The two subsystems are structured as feedback loops. PA1 b) a subsystem B) that uses the signal at the input of the subsystem C) and includes the circuit (placed upstream of subsystem A1) used for sampling the signal entering the system and converting it into a digital one. Subsystem B) is based on a maximum power algorithm; this implies that its operation is independent of the behaviour of subsystem A) (because the algorithm is insensitive to the phase of the processed signal and therefore to errors in reconstructing phase and frequency of the carrier) and also independent of subsystem C) (because the algorithm does not use the estimated symbols). PA1 c) a subsystem C) comprises a fixed digital filter which completes the shaping of the transmitted pulse and an adaptive digital equalizer. For both, the structure of Finite Impulse Response (FIR) filters has been chosen because they are particularly easy to implement. The equalizer coefficients are updated through an algorithm insensitive to the phase of the incoming signal so that its convergence is independent of the carrier recovery carried out by subsystem A). PA1 it is not necessary to use a voltage controlled oscillator (VCO) to carry out the baseband conversion, but a free oscillator is sufficient; PA1 both the subsystems and their interconnections maintain their operation and effectiveness in a wide range of low/medium capacity links; and PA1 single components and the demodulator as a whole are suitable for a wholly digital implementation that minimizes interconnection problems of analog and digital functional blocks; moreover the digital realization along with adaptivity of the subsystems make the trimming of the apparatus much less critical during its production.
The problem of tolerance and instability of the free oscillators is difficult to solve for small capacity links for which the symbol frequency is lower and therefore the ratio of the frequency error to the symbol frequency itself is higher.
If, on the other hand, links having capacity increasing gradually are considered, the spectral occupation or the number of modulation levels and hence the sensitivity of the system to selective fading are increased. Consequently the introduction of an adaptive equalizer for counterbalancing the effects becomes necessary. This device constitutes a countermeasure against fading, but at the same time it is able to correct the linear distortions inevitably introduced in the realization of the system because of non-idealness of components. Therefore the introduction of the equalizer makes the system more robust with respect to anomalies in propagation, and also implies a less critical trimming; for this reason an equalizer (even if with few coefficients) is also desirable in the design of a low capacity system.
Moreover, in any demodulation system, it is necessary to insert a device for recovering the clock synchronism phase and frequency. For the correct operation of the system, it is convenient that the algorithm for the clock recovery be independent of the behaviour of devices for carrier recovery and adaptive equalization.
In a digital modem, the clock synchronism is used to drive the signal sampling circuit with a suitable frequency, which is a multiple of the symbol frequency.
It is an object of the present invention to provide a solution for the above-mentioned problems having, inter alia, the advantages mentioned at the outset.